1. Field of the Invention
The present invention concerns a field effect transistor made of materials of the III-V group such as GaAs and AlGaAs, having a double heterojunction. Each heterojunction is made so that a two-dimensional gas of charge carriers can appear at the interface, but the types of doping of the semi-conducting material layers (n and p) forming these heterojunctions are chosen so that the transistor has an n channel and a p channel which are used simultaneously or individually.
2. Description of the Prior Art
Field-effect transistors of the TEGFET (two-dimensional electron gas field effect transistor) type are now well known, and their theory was established in 1979. These transistors consist essentially of a heterojunction between a material with a small forbidden gap, GaAs for example, which is non-intentionally doped (NID) or weakly n doped or p doped, and a material with a wide forbidden gap, AlGaAs for example, which is n type doped. Under certain conditions (such as the adaptation of the lattice parameters, the thickness of the layers, the doping levels, etc.), there appears a highly mobile two-dimensional electron gas with a heterojunction on the side of the material with a narrow forbidden gap.
These transistors generally use an n channel because the electrons are more mobile than the holes and hence more useful for working in microwave frequencies, namely at gigahertz.
However, to process AC signals or in complementary logic circuits (for example, an n channel transistor and a p channel transistor as in complementary MOS transistors) it is worthwhile to have a transistor that has both n and p characteristics.
This is what is proposed by the invention which combines two parallel channels in a single transistor. These are an n channel in which there is a two-dimensional electron gas and a p channel in which there is a two-dimensional hole gas. These two channels can coexist and work in turn or, depending on the value of the voltages applied and the nature of the contacts providing access to the said channels, they may be selected to choose one of the two channels.
Thus, the transistor according to the invention has a substrate and, if necessary, one or more buffer layers for adapting the lattice parameters. On this support, an active region consists of three layers of semi-conducting materials of the III-V group forming two heterojunctions between them, the intermediate layer having a small forbidden gap and being non-intentionally doped. The first and third layers are made of materials with wide forbidden gaps. One of them is n doped and the other is p doped at doping levels of the order of 10.sup.17 -3.10.sup.18 at.cm.sup.-3, such that a two-dimensional electron gas can appear at a first heterojunction and a two-dimensional hole gas can appear at a second heterojunction.